The present disclosure relates to gas sensor devices. More particularly, the present disclosure relates to gas sensor devices for harsh environments.
Nitrogen oxides, or NOx, is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as a yellowish or reddish-brown layer over many urban areas. Other oxides of nitrogen are also important species which may require detection and monitoring such as nitric oxide (NO) and nitrous oxide (N2O).
Nitrogen oxides form when fuel is burned at high temperatures, as in a combustion process. Examples of sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.
It is well known that in recent years, organizations like the EPA (Environmental Protection Agency) and the ICAO (International Civil Aviation Organization) have implemented regulations that limit the amount of pollutants emitted into the troposphere by fossil-fuel powered devices such as gas turbines, aircraft engines, trucks, and locomotives. For example, the EPA invoked a series of “tiered” regulations limiting the nitrogen oxide (NOx) production, among other effluents, emitted from diesel locomotives. In 2000 (Tier 0), a diesel locomotive was allowed to emit 9.5 gm/hp-hr of NOx emissions. However in 2005 (Tier 2), such engines are limited to only 5.5 gm/hp-hr of nitrogen-based pollutant, approximately one half of Tier-0 concentrations. These stringent regulations have forced manufacturers to rapidly develop new low emissions combustion technologies. Additionally, the laws have also had a synergistic effect. Not only do manufacturers want to limit emissions, but employ the concentration of specific exhaust products to actively control the power-generation process. In other words, because the combustion of hydrocarbon-based fuels is truly a thermo-chemical process, the ideal engine “health monitor” is found in the effluent gases. Similar to modern automobiles that employ an oxygen sensor in the exhaust stream to control the fuel-to-air ratio, industrial power generation companies require related sensor technologies to control and diagnose engine performance, albeit on a larger scale.
NOx and other exhaust gas species are difficult to sense and control, especially in harsh environments such as automobile, diesel, aircraft, and locomotive exhaust streams; power generation; flue gases; gas turbines. Such harsh environments can often reach temperatures of 300 degrees Celsius (° C.) to 1,000° C. These environments also often have corrosive atmospheres containing gases such as hydrocarbons, NOx, and SOx. These harsh environments may have high vibrations and high pressures, alone or in combinations with the high temperatures and/or corrosive atmospheres. Current solid-state gas sensors cannot operate in these harsh environments unless supplementary cooling of the gas-sampling probe is provided. Other gas sensors that are electrochemical based are expensive and cannot withstand the high temperatures that are present in these harsh environments. These sensors often times do not offer the accuracy required to meet many of the EPA emission regulations. Most ceramic-based sensors have difficulty or do not function at all at temperatures below 500° C.
Accordingly, there is a need for gas sensor devices for sensing and monitoring exhaust gases and other harsh environment gases that can operate or withstand over a wide range of temperatures minimally from room temperature to above 600° C. Such gas sensors must have acceptable full-scale range, measurement resolution, and signal-to-noise ratio.